The invention relates to replacement of tubes in power amplifiers with solid state devices. In particular, the invention is directed to a solid state circuit that duplicates tube power amplifier compression.
Tube compression occurs whenever the tube power amplifier is driven into hard clipping. Normally, a solid state amplifier driven into hard clipping creates harsh odd-order harmonic distortion (square waves). In contrast, a tube amplifier compresses the signal so that the level decreases and it does not sound as harsh and strident. As a result, the sound is more subdued, but still has what the players call "punch". Thus, compression is a musical function that gives a tube power amplifier an edge over conventional solid state power amplifiers according to most heavy metal and bass guitar players, particularly at clipping conditions.
The foregoing is a non-technical description of a phenomenon called increased crossover distortion. This function happens in all tube power amplifier designs whenever the output tube grid is driven positive with respect to the cathode causing it to become simply a forward biased diode.
In a typical push-pull configuration, using two class-B biased tubes, the diode in each push-pull output stage causes the average bias level to increase at high signal levels and forces the class-B biased tubes to become over biased. Such condition causes the output signal to have severe crossover distortion, a condition where the signal zero crossing is delayed significantly.
A typical tube power amplifier 10 which has been used on many popular models, is shown in FIG. 1. Typical circuit operation is described below followed by a description of overload (or tube compression) conditions.
In FIG. 1, input signals are coupled via coupling capacitor 11 to the grid of vacuum tube 12 (e.g., 12AX7), which with tube 14 is half of what is called a long tailed phase inverter circuit. In this circuit, the cathodes of tubes 12 and 14 are connected together, as shown. Thus, tube 12 operates in a grounded cathode mode; while tube 14 operates in a grounded grid mode with respect to the input grid of tube 12. Accordingly, equal but out-of-phase signals appear at the plates of 12 and 14. The purpose of the phase inverter is to supply two out-of-phase signals to class-B biased push-pull output tubes 16 and 18.
Cathode resistor 20 sets the bias for each tube 12 and 14. Grid resistors 22 and 24 are the respective grid bias resistors. Resistor 26 is a common cathode resistor. Resistor 28 is used to introduce feedback from the output to reduce overall distortion. The grid of tube 14 is shunted to ground (in this case, the low impedance feedback point) via capacitor 30, as is necessary for grounded grid operation. Load resistors 32 and 34 are the respective plate loads for tubes 12 and 14. The plate signals are coupled to the output tubes 16 and 18 via capacitors 36 and 38.
Each output tube grid is connected to a negative bias source (e.g., -55 V) via bias resistors 40 and 42. This -55 V sources is generated externally from this circuit and is filtered adequately by capacitor 44. Negative 55 volts is chosen as the appropriate value to bias the output tubes 16 and 18 (e.g., 6L6GC) into good class-B operation with minimal crossover distortion at low signal levels.
Completing the circuit, resistor 46 is a feedback resistor; resistors 48 and 50 are power supply decoupling resistors; capacitors 52, 54 and 56 are filter capacitors for the various supply sources in the B+circuit. Finally, transformer 60 is a conventional tube push-pull output transformer, in this case with output taps for 8 and 4 ohms. The power amplifier 10 delivers approximately 50 WRMS to the matching load value.
At all signal levels below output clipping (the output waveform being clean and free of distortion), the signal levels at the grid of each output tube 16 and 18 is well below 55 volts peak swing, and the average DC bias level at each output tube grid is -55 VDC. However, at clipping and beyond, the signal levels at each output tube grid will exceed 55 volts peak swing. Thus, the grid will be biased positive with respect to the cathode at each positive peak signal swing. Whenever the grid is driven positive with respect to the cathode, it becomes a simple forward biased diode. With the positive peak swing clipped, the average negative DC bias voltage level at the grid of each output tube 16 and 18 is increased in proportion to the overload input value above the clipping value. Thus, the output tubes 16 and 18 become over biased beyond class-B and at severe output clipping significant crossover distortion is generated as well. Consequently, at overload, the output signal of tube amplifier 10 will be clipped at the peaks. However, it will not be as "dirty" as a typical solid state power amplifier operating under the same conditions, because a large portion of the overloaded output waveform is forced or compressed into the severe crossover distortion region. To a musician, such a waveform is much more musical in nature and "cleaner" (i.e., less harsh) than a solid state amplifier at overload. Due to the compression (i.e., distortion near the zero crossover), the actual peak output clipping is reduced and is far more tolerable than that of the solid state amplifier. This phenomenon is thus, tube power amplifier compression.